The present invention relates to an air conditioner or refrigerating plant incorporating a scroll-type refrigerant compressor which is driven by a variable-speed driving system such as a driving system having an electric motor which is controlled through an inverter. More particularly, the present invention is concerned with an air conditioner of the type mentioned above, wherein the scroll-type refrigerant compressor is provided with a bypass valve which is intended for preventing refrigerant form being excessively compressed in the compressor.
Air conditioners incorporating scroll-type refrigerant compressors have been known. In general, a scroll-type refrigerant compressor (referred to as "scroll compressor" hereinafter) has an orbiting scroll member constituted by an end plate and an integral scroll wrap having an involute or a similar spiral form and protruding from one side of the end plate. The scroll compressor also has a stationary scroll member having a construction substantially the same as that of the orbiting scroll member and provided with a discharge port in the center of an end plate. The orbiting scroll member and the stationary scroll member are assembled together such that their warps mesh each other and are encased in a hermetic housing which is provided with a suction port. The orbiting scroll member is held on the housing through an Oldham's mechanism in such a manner that it cannot rotate about its own axis but its center can orbit or revolve around the center of the stationary scroll member. The revolution of the orbiting scroll member is caused by a crankshaft which is driven by an electric motor. During orbiting of the orbiting scroll member, closed spaces constituting compression chambers are successively defined by the end plates and the wraps of both scroll members and are progressively moved towards the center of the stationary scroll member while decreasing their volumes and are finally brought into communication with the discharge port in the center of the stationary scroll member. In consequence, a refrigerant gas sucked by and confined in each compression chamber is progressively compressed to a pressure higher that the suction pressure and the thus compressed refrigerant gas is discharged through a discharge port.
FIGS. 2A and 2D illustrate sucking and compressing operations performed by the scroll compressor during one full revolution of the orbiting scroll member, i.e., in the period in which the center of the scroll wrap 6b of the orbiting scroll member revolves counterclockwise from a position shown in FIG. 2A to the position shown in FIG. 2D. Referring to FIG. 2A, a space denoted by 12a formed between left portions of both scroll wraps as viewed in FIG. 2A is still open and its size is still increasing so that a refrigerant gas is sucked by this space 12a. In the state shown in FIG. 2B, the space is closed to form a closed space, i.e., compression chamber, of the maximum volume. In this state, the suction of the refrigerant gas has been completed. In the state shown in FIG. 2C, the compression chamber now denoted by 12c has been contracted and is just going to be brought into communication with the discharge port 13 in the center of the stationary scroll member. Namely, in this state, the gas in the compression chamber 12c has been fully compressed and is just going to be discharged. As the orbiting scroll member further revolves to the position shown in FIG. 2D, the closed space or compression chamber now represented by 12d is brought into communication with the discharge port 13 so that the refrigerant gas compressed in this compression chamber 12d is discharged through the discharge port 13.
The ratio of the pressure of the gas immediately before the discharge, i.e., the pressure in the compression chamber 12c shown in FIG. 2C, to the suction pressure, i.e., the pressure in the chamber 12b shown in FIG. 2B, has a constant value which is determined by factors such as the design of the scroll wraps. This ratio, expressed by (pressure immediately before discharge/suction pressure) is referred to as "design pressure ratio" of compressor. On the other hand, the value of the ratio of the pressure in the discharge port 13, i.e., the discharge pressure, to the above-mentioned suction pressure varies depending on the state of operation of the air conditioner. This ratio, expressed by (discharge pressure/suction pressure), is referred to as "operation pressure ratio".
FIG. 3 is a diagram showing the relationship between the revolution angle of the orbiting scroll member and the internal pressure of the compression chamber. At a point indicated by "A", the suction of the gas is completed in one of the compression chambers. When the orbiting scroll member has been revolved to a position shown by "B", the compression in this compression chamber is completed, thus attaining the design pressure ratio of the compressor. A further revolution of the orbiting scroll member brings this compressionchamber into communication with the discharge port so that the compressed gas is discharged. Then, after sucking the gas, another compression chamber is defined again when the orbiting scroll member has completed one full revolution to the position "A". This operation is cyclically repeated so that the gas is sucked, compressed and discharged. When the operation pressure ratio is smaller than the design pressure ratio of the compressor, the pressure in the compression chamber varies in such a manner as to follow a curve ABC. This state of compressing operation is referred to as "over-compression". It will be seen that the portion of the compressing work corresponding to the area DBC is useless. Conversely, when the operation pressure ratio is greater than the design pressure ratio, the pressure in the compression chamber varies in such a manner as to follow a curve ABE. In this case, the compressor is required to perform an additional work corresponding to the area BEF.
Obviously, the over-compression wastefully consumes the power or work, so that it should preferably be minimized. It has been known to provide a bypass valve as means for preventing such over-compression. FIG. 4 shows a known arrangement of such a bypass valve. This arrangement has a bypass port 21 formed in the end plate of the stationary scroll member so as to provide a communication between the compression chamber and a discharge chamber, and a check valve provided on the discharge side of the bypass port 21 so as to prevent any reversing of the gas from the discharge side into the compression chamber. The bypass port 21 is provided in the close proximity of the wrap 5b of the stationary scroll member. Thus, the bypass port 21 is covered by the wrap of the orbiting scroll member so that it does not communicate with a compression chamber until the compression stroke of this compression chamber caused by the revolution of the orbiting scroll member proceeds to a predetermined degree. However, as the compression stroke proceeds beyond this degree, the bypass port 21 is again opened to communicate with the compression chamber. In FIG. 3, a straight line GH represents the angular range of revolution the orbiting scroll member in which the bypass valve 21 is held in communication with the compression chamber.
When the compressor operates in accordance with the curve shown in FIG. 3 under the operation pressure ratio indicated by C, the pressure in the compression chamber varies in accordance with a curve ADC because the discharge of the gas is commenced at the point D. It will be understood that the discharge of the gas does not take place in the region between points G and D due to the presence of the check valve. In consequence, the wasteful over-compression is completed. Thus, the provision of the bypass valve, having a bypass port which opens in the region GH, can effectively prevent over-compression at least when the operation pressure ratio is I or greater. The timing at which the bypass port 21 is brought into communication with the compression chamber is determined by the position of the bypass port 21.
An example of the known scroll compressors having the described bypass valve arrangement is disclosed in U.S. Pat. No. 4,389,171.
A description will be made hereinunder as to the difference between a known air conditioner incorporating a compressor driven at a constant speed and a known air conditioner which employs a compressor the speed of which is varied, as is the case of the air conditioner of the present invention, by a variable-speed driving system such as that incorporating an electric motor controlled through an inverter.
In the air conditioner employing a constant speed compressor, the compression capacity of the compressor is constant so that the compressor has to be repeatedly started and stopped when the level of the load is smaller than the capacity of the compressor. When the compressor is stopped, a substantially equilibrium state of pressure is attained in the refrigerant circuit of the air conditioner, whereas, when the compressor operates, a pressure difference is established across the compressor so as to form a high-pressure portion and a low-pressure portion in the refrigerant circuit. This means that an additional work is required for the compressor because it has to establish the pressure difference from the equilibrium state each time it is started. In order to obviate this problem, air conditioners have been proposed in which the driving speeds of the compressors are varied such that each of the compressors is operated at a higher speed when the level of the load is high and at a lower speed when the load level is low. Since the internal volume of the refrigerant circuit is constant, the higher operation speed of the compressor, i.e., a greater rate of discharge of the refrigerant, increases and decreases the discharge pressure and the suction pressure, respectively, thus attaining a high operation pressure ratio. Conversely, a lower operation speed of the compressor, i.e., the smaller rate of discharge of the refrigerant gas, causes the discharge pressure and the suction pressure to be decreased and increased, respectively, thus establishing a low operation pressure ratio.
Although scroll compressors having bypass valves have been known, no suggestion has been made to the position of the bypass port from the view point of efficiency of the compressor. For instance, the bypass port disclosed in the aforementioned U.S. Pat. No. 4,389,171 is intended for reducing the load applied to the compressor when it is started up. Also proposed is a scroll compressor in which a bypass valve is provided so as to relieve the compressed gas when the pressure of the gas compressed in the compression chamber has been increased beyond the discharge pressure. In this known compressor, however, the position of the bypass valve is not definitely determined from the view point of efficiency of the compressor.
Thus, the prior art proposed hitherto fails to suggest positioning of the bypass valve for the purpose of inproving the efficiency of operation of the compressor and, hence, of the air conditioner.